This section introduces aspects that may be helpful in facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admission about what is in the prior art.
So-called massive MIMO antenna arrays, which consist of a large number (e.g. more than 20) of antenna elements, are seen as a solution for handling the exponentially growing mobile data traffic. Each antenna element of a massive MIMO antenna array may be connected to a dedicated low power transceiver (e.g. in the double-digit milliwatt range), which provides a corresponding transmit signal to the antenna element.
On the one hand, such massive MIMO antenna arrays provide a large capacity in terms of bits/s/Hz. But on the other hand, operating each antenna element with a dedicated low power transceiver may increase the energy consumption of the massive MIMO antenna array in comparison to conventional 2×2 or 4×4 antenna arrays. Therefore, an efficient signal generation may be required in each low power transceiver. An important metric in this context is the overall energy efficiency, which may be measured for example in bit per Joule.
Even if a current data throughput is quite low, it may be necessary for the operation of the massive MIMO antenna to send a transmit signal from all the antenna elements in order to maintain a pre-configured transmission pattern (cell coverage) and to achieve a minimum signal strength at all locations of the cell coverage.
Additionally, currently intended massive MIMO deployments require a signaling cell or overlay cell in addition to a radio cell provided by the massive MIMO antenna array because the massive MIMO antenna array cannot easily be used for an attachment procedure of a mobile station due to the restriction that a radio channel to a respective mobile station has to be sounded in order for the base station to transmit a massive MIMO beam.
The overall energy efficiency of the massive MIMO antenna array is greatly reduced in low-load situations, because of the overhead energy consumption of the low power transceivers with electrically driven components such as DAC (DAC=Digital-to-Analog Converter), modulator, LO (LO=Local Oscillator), drivers, pre-amplifier, power amplifier etc. Such overhead energy consumption is usually to a wide extend independent of a current output power.